Resistive random-access memory (RRAM or “resistive memory”) is a type of non-volatile memory where the data storage function is implemented in a variable resistance element whose resistance value can change between a low level and a high level. For example, most resistive memory devices include as the variable resistance element a controllable resistor material between upper and lower conductive electrodes. The controllable resistor material may be a transition metal oxide or other suitable materials. A conductive path is created or dissolved in the controllable resistor material, corresponding to low-resistive and high-resistive states. A resistive memory cell in a resistive memory device typically includes a variable-resistance resistive memory element connected serially with a switching device acting as a cell selector. The switching device is typically an NMOS transistor.
In some examples, a resistive memory cell can store one logical state by programming the resistive memory element to have a relatively large resistance. The resistive memory cell can store a second logical state by programming the resistive memory element to have a relatively small resistance. However, due to limitations in the fabrication process, the resistance values for the high and low resistance levels are often distributed over a range. Furthermore, the resistance values for the high and low resistance levels often vary over time.
Resistive memory devices have advantages over existing memory devices, such as DRAM, for their low operating current, fast access time, and long data retention. Furthermore, resistive memory devices are attractive as replacement for existing memory devices because of their compatibility with existing CMOS fabrication technologies.